The present invention relates to a device for injection of reducing agents into a shaft furnace, in particular for injection of pulverised coal into a blast furnace during production of pig iron.
To save high-quality reducing agents such as coke in the production of liquid metals in shaft furnaces, a portion of these reducing agents can be replaced by pulverised coal. The pulverised coal is obtained from raw coal in a preparation plant. The raw coal is crushed and dried and subsequently stored temporarily in coal silos. For introduction into the shaft furnace the temporarily stored pulverised coal is loosened, pressurised and injected into the shaft furnace pneumatically by a carrier gas via conveying lines. Injection is generally effected by several injection lances, which terminate in the blast tuyeres of the shaft furnace, so that introduction takes place simultaneously at various points of the shaft-furnace.
To achieve the largest possible saving of reducing agent costs by the injection of pulverised coal, the injected pulverised coal must be converted as completely as possible in the blast tuyere air duct, so that only residual coke need be gasified in the eddy zone. The term xe2x80x9ccomplete conversionxe2x80x9d here means that all carbon atoms combine with oxygen, carbon monoxide and/or carbon dioxide being formed. If the conversion in this zone is not complete, which may be the case in particular at high injection rates, conversion residues are concentrated in the shaft furnace, which leads to unstable furnace conditions.
The important causes of the defective pulverised coal conversion in the eddy zone lie firstly in the small dimensions of the actual reaction space and secondly in the high speeds of the media and flow properties of the hot blast air in the tuyere. Within the short time consequently available for the complete conversion of the coal particles at the lance outlet the pulverised coal flow emerging from the lance must be mixed with the hot-blast air, the individual pulverised coal particles must be heated to such an extent that their ignition temperature is achieved,. Released pyrolysis gases must be mixed with the available oxygen, ignite and the solid residue after conclusion of the pyrolysis must enter into an oxidation reaction with any still free or bonded oxygen.
To improve the reaction conditions for the conversion of pulverised coal in the injection zone and thus accelerate the reaction kinetics of the conversion, various measures have been proposed, e.g. increase of the oxygen concentration in the hot blast air, the local increase of the oxygen concentration by simultaneous injection of oxygen via coaxial lances or double lances or the minimisation of the pulverised coal outlet pulse at the tip of the lance by enlargement of the outlet cross-section. Although these measures, some of which are already used, whereas others are only in the testing or optimisation phase, bring about a certain improvement in the reaction conditions, the acceleration of the pulverised coal conversion which is achieved by these measures has proved to be still inadequate.
Consequently, the task of the present invention is to propose a method and device for injection of coal into a shaft furnace which substantially accelerate the reaction kinetics of pulverised coal conversion, so that the latter begins immediately after the injection and is essentially concluded on entry into the eddy zone.
According to the invention this problem is solved by a method for injection of reducing agents into a shaft furnace according to claim 1 and by a device according to claim 6.
In the method according to the invention, in which a reducing agent such as pulverised coal is conveyed in a pneumatic flow to the blast furnace, the conveying flow is divided into several partial flows, which are led through a heating device, the reducing agent in the individual partial flows being heated inside the heating device by a heat supply. The individual partial flows are subsequently preferably combined into a common conveying flow again for homogenisation of the temperature conditions prior to a possible apportionment. of the conveying flow to the individual injection lances distributed around the shaft furnace.
By heating the pulverised coal inside the heating device heat can be fed to the pulverised coal in a controlled manner, so that its temperature can be set to a value which is favourable for use of the pulverised coal conversion in the shaft furnace. Consequently the pulverised coal preheated in this way needs to absorb significantly less heat from the hot blast air after injection into the shaft furnace in order to achieve its ignition temperature, and its conversion in the shaft furnace clearly starts more quickly than with pulverised coal injected xe2x80x9ccoldxe2x80x9d, so that the short available reaction time can be fully utilised for the conversion.
In addition the cooling of the reaction space is reduced by the smaller heat transfer of the hot blast air to the pulverised coal with the result that it can be ensured that the temperature in the entire reaction space remains high enough to permit reduction of the carbon dioxide formed during conversion of the pulverised coal to carbon monoxide. Consequently a further increase in the pulverised coal conversion is achieved, because the oxygen atoms present can bond substantially more carbon atoms. On the other hand the higher carbon monoxide proportion has a highly favourable effect on the operation of the shaft furnace, because it serves as a reducing agent for the actual metal recovery.
In a preferred embodiment the reducing agent is heated in several stages, i.e. subdivision of the conveying flow into several partial flows, conduction of the individual partial flows through a heating device and heating of the reducing agent in the individual partial flows inside the heating device are repeated several times, the individual partial flows between two successive stages being combined into a common delivery flow for homogenisation of the temperature conditions.
To improve the heat transfer to the reducing agent in a partial flow, the latter is preferably made to rotate about its direction of flow. The rotary motion and the associated turbulence in the partial flow produce rearrangement of the material in the partial flow, so that the latter is thoroughly mixed. In addition the velocity of the individual particles in the partial flow and their exchange path length in the heat-exchanger increase. Consequently the heat transfer to the reducing agent in the heat-exchanger can be clearly improved. Hence all partial flows are preferably made to rotate in this way.
Finally cold reducing agent can be fed to the common conveying flow for control of the injection temperature of the reducing agent before the injection into the shaft furnace.
A device according to the invention for injection of reducing agent into a shaft furnace comprises a conveying line for pneumatic conveyance of the reducing agent to the shaft furnace, several heat-exchanger tubes integrated in the conveying line with parallel connection, so that the pneumatic conveying flow is divided into several partial flows, and a heating device for transfer of thermal energy to the individual partial flows. With this device heat can be fed in a controlled manner to the reducing agent during its conveyance between a storage tank and the injection lances on the shaft furnace.
The heating takes place inside the conveying line, i.e. it can be carried out immediately before the injection of the pulverised coal into the shaft furnace. Consequently the pulverised coal is discharged to the consumer immediately after the heating, so that safety problems do not occur during temporary storage of a heated fuel. In addition the heat losses to the environment are very small, not least because of the reduced radiation surfaces.
Subdivision of the pneumatic conveying flow into several partial flows produces the required effective heat transfer surface in an arrangement with small dimensions, i.e. the surface at which heat is transferred to the reducing agent or the respective partial flow. In addition a high coefficient of heat transfer is established in the solid/conveying gas mixture as a result of the turbulent flow and the higher gas density because of the excess conveying pressure.
Consequently the individual partial flows can absorb large quantities of heat in a very short time and over a short distance, so that the length of the heat-exchanger tubes can be kept correspondingly short. The entire device is accordingly characterised by its compact design, which may be highly important, e.g. during conversion of existing plants.
In a preferred embodiment the heating device comprises a heating chamber, which can admit a heat carrier, the heat-exchanger tubes being arranged inside the heating chamber or extending at least partially through the heating chamber. The heat carrier may comprise, for example, a hot gas from a furnace. Direct heat transfer between the hot gas and the solid/conveying gas mixture then takes place. As a result of the high temperature of the hot gas a large mean temperature gradient of hot gas to solid/conveying gas mixture providing the heat exchange can be maintained. In addition a significant heat transfer due to radiation is added to the convective heat transfer on the hot gas side because of the (sometimes) high temperatures in the hot gas, so that an extremely high heat flow density is achieved.
The furnace is preferably operated with weak gas, e.g. blast furnace gas. Consequently unfavourably high combustion temperatures, which would necessitate precooling of the hot gas before entry into the heating chamber during normal operation, are avoided. The high waste gas temperature prevailing behind the heating chamber during operation with a high mean temperature gradient of hot gas to solid/conveying gas mixture does not constitute a significant economic loss if there is no further utilisation of the residual heat of the waste gas.
If the heat-exchanger tubes are not exposed to a conveying gas/solid mixture, e.g. when the hot gas side is started up, the hot gas can be brought to a lower temperature by admixing cold air behind the furnace; the heat-exchanger tubes are thus protected against overheating. The waste gas can be cooled to a permissible discharge temperature in exactly the same way by admixing cold air, if the device is operated with a high mean temperature gradient of hot gas to solid/conveying gas mixture and the waste gas is to be discharged directly into the environment after leaving the heat-exchanger without further utilisation of the residual heat.
Alternatively the heat carrier may comprise a liquid or condensing medium. A liquid heat carrier has a higher specific heat capacity Cp than the gaseous one, so that a larger quantity of heat per volume can be entrained and emitted in this case. A condensing medium, i.e. a medium which undergoes a phase transition during the heat emission, is characterised by high heat emission at a constant temperature. Hence a medium with a condensation temperature coinciding with the required injection temperature of the reducing agent is preferably selected. Consequently overheating of the reducing agent, e.g. if a heat-exchanger tube is blocked, can be largely precluded.
The heat-exchanger tubes are advantageously arranged essentially vertically and are traversed from the bottom upwards by the partial flows. Consequently solid deposits with local overheating of the tubes are avoided, the tube cross-section is exposed to a uniform flow in each case, and the heat transfer from the tube wall to the solid/conveying gas mixture flow is optimised.
In an advantageous embodiment of the device a swirler, which causes the partial flow passing through the respective heat-exchanger tube to rotate about its direction of flow, is arranged in one or more, but preferably in all heat-exchanger tubes. Rotation of the partial flow improves the thorough mixing of the respective partial flow, so that the heat transfer to the partial flow is improved. In addition the velocity of the individual particles in the partial flow as well as their exchange path length in the heat-exchanger increase.
The swirler may, for example, comprise one or more spiral metal strips extending in an axial direction inside the heat-exchanger tube. The pitch of the spiral twist of the respective metal strip can be selected according to the required exchange path length of the reducing agent in the heat-exchanger.
In a preferred embodiment of the invention the heat-exchanger tubes are assembled a certain distance from each other to form a heat-exchanger nest, the latter having on its inlet side a distributor for uniform apportionment of an incoming conveying flow to the individual heat-exchanger tubes and on its outlet side a collector for bringing together the individual partial flows to form an outlet flow. The distributor and the collector are preferably of identical construction so that the direction of installation of the heat-exchanger nests is unimportant. Consequently the heat-exchanger tubes are combined to form easily assembled, standardised units and blocked heat-exchanger nests can easily be changed if required.
In the case of conveying lines with a large cross-section the division of the conveying flow into several partial flows can take place in several stages. The conveying flow is apportioned, for example, via pre-distributors to several heat-exchanger nests. The device accordingly comprises several heat-exchanger nests, the latter preferably being assembled a certain distance from each other to form a heat-exchanger group, the latter having on its inlet side a pre-distributor for uniform apportionment of an incoming conveying flow to the individual heat-exchanger nests and on its outlet side an additional collector for bringing together the individual partial flows to form a common outlet flow. The pre-distributor and the second collector are preferably of identical construction and are, for example, of the type of the particle flow distributor described in U.S. Pat. No. 4,702,182.
In a preferred embodiment of the invention several heat-exchanger nests or groups are connected in series. In this way a compact arrangement can be achieved. The solid/conveying gas mixture partial flows are brought together behind each heat-exchanger nest or group, and any different temperature increases that might occur in the individual tubes or tube nests are compensated. The failure of individual tubes or tube nests does not significantly impair the serviceability and exchange capacity of the entire device. In a plant with heat-exchanger groups individual nests can be taken out of service for maintenance or shutdown, as long as shut-off valves are provided.
Guide vanes are advantageously arranged inside the heating chamber in such a way that effective flow of the heat carrier to the heat-exchanger tubes takes place. The guide vanes may, for example, be baffles, which divert the hot gas flow selectively to the individual heat-exchanger nests.
A bypass line, which terminates in the conveying line behind the heat-exchanger tubes when viewed in the direction of the conveying flow, should preferably be provided for each conveying line. This allows the conveyance to be continued smoothly in the event of failure of the heat-exchanger or if heating is dispensed with. The change-over between heat-exchanger and bypass line can be carried out gradually and smoothly, with minimisation of the risk of line blockages, by the use of solid mass flow control valves.
If several solid/conveying gas mixtures are heated in parallel, whereby the total hot gas supply emanates from a furnace, different heating temperatures can be established in the individual solid/conveying gas mixtures with the aid of the solid mass flow control valves.